By saying that one lacks confidence, what does one really mean?
Usually, in my experience, this lack of confidence is a fear of rejection, a fear of criticization. A writer can't write if they don't first believe in their work, but they can get psyched out when it comes to actually exposing it to the world.
I want to be clear that it is not a bad thing to be afraid of these things: being afraid means that you want to do well, which is the foundation to any career, the drive to do it. It is just when you refuse to fully digest a reviewer's complaints where things spiral.
To this, all you can do is take the plunge and grow in the process.
As for whether or not your book is good, it depends on what you want that "good" to reflect in your novels. If you are an absolute newbie to novels, I recommend defining that "good" as grammar and diction quality. Assuming that you became a writer because you have a story in your head that you want to share, it is best to make sure that you know how to properly translate those thoughts through language and its funny little rules.
The good news is that language is a natural trait that humans develop and can therefore be practiced simply by being exposed to it. That said, it is good to read, read, and read more than you talk and scroll down social media. Verbal speech and social media both use informal grammar that only make sense in the context that they are being spoken in; becoming accustomed to this and accepting this sort of speech will cripple your writing in the eyes of your readers. Reading more things -- books, magazines, comics, video transcripts, youtube captions -- will help you visualize your thoughts in words as you are exposed to more and more phrases. TLDR; get familiar with the written language you are writing in!
Now, you could also properly study grammar independently, but I am the sort of person who reads through things once to get the gist and use the material immediately, so I am nowhere near an expert on how to learn grammar 😅 . I do recommend it, though, since there are school materials out there for free.
If you think you can depend on an editor to fix your grammar oopsies...yes, you can, but your story will be convoluted. Why? Because language is the vessel that your story comes in. See, grammatically, the word "but" should primarily not be used at the front of a sentence because it is usually used as a coordinating conjunction or a predicate, which basically means that "but" is a word that functions as a bridge that requires two sides -- the start of a sentence does not provide two sides, thus being grammatically incorrect.
However, there are many authors who forgo this rule and use "but" at the start of some sentences. Why? Because they know that this discrepancy in grammar, which any reader is instinctually aware of, will catch people's attention and make the sentence more dramatic, more captivating.
...Hang on, this isn't a grammar lecture. You asked how to build confidence in your craft.
A newbie's biggest flaw in writing after grammar is their inexperience. And their tendency to delete and backpedal. After the insecurities of grammar, which your brain will automatically recognize but not tell you because it is a bully, there is the insecurity about the lack of experience. The only thing to do is to write without deletion and to fail several times.
In fact, I don't know if there will ever be a sense of success at the end of this tunnel; you just gotta write and know that in doing so, you are getting better. You just can't see it yourself.
It's like growing taller without realizing it, you know, how relatives go, "Oh, little baby so tall now, haha"? It's all because they have not watched your progress and only know that you were once a toddler and now a thirty year old. You, on the other hand, still feel like you're nineteen and trying not to open your mouth while chewing your food next to your crush's parents, but the truth is that you are thirty and you have a new crush now who's actually your legal spouse. Like, you got a paper and everything, good for you.
...I'm sorry, I was losing focus and starting laughing at nothing for about five minutes after I wrote "good for you". Don't worry, this is just because I'm tired 😄 .
That said, I think that I've written more than enough. This sort of hobby/job takes a lot more willpower than others in that you must constantly motivate and force yourself to be proud in your own hard work. Confidence isn't just something to have, and I'd even argue that this confidence that people normally talk about is a personality trait instead of a learned one.
You can't know if your work is good or not if you never publish anything. You can't publish a work if you never write anything. You can't keep writing if you keep deleting. Don't be a machine gun, be a big ol' stick that a dog dropped off a cliff with no bottom: a machine gun, though full of content, will not do anything until the trigger is pulled. A beautiful lil' stick, on the other hand, has no choice but to fall and land.
I hope that my word vomit helped you, even if it's just a tiny bit! Keep on writing! I look forward to your story.